Tuesday, August 25, 2009

The concept of ride comfort varies from person to person. If one were to ask 10 different people about a particular bike's ride characteristics, its likely they'll say 10 different things. There's probably a good reason for this. Physiologically, we can sense pain better than comfort because our bodies have lots of pain receptors (nociceptors) but there's little evidence of a comfort receptor. So our bodies are built without 'signal probes' for comfort. Therefore we tend to call something comfortable if there's no discomfort, i.e, if our nociception does not pick up discomfort signals. (If there's a more involved perception mechanism than what I've described, its outside the scope of this blog)

But even the perception of discomfort varies from person to person. A seasoned veteran testing out a bike is likely to have a different perception of discomfort on a given bike than a beginner may. Bicycle marketing literature as well as reviews of bikes usually are plentiful in these sort of subjective feelings that no one can put a number upon. X person tests the bike. He likes it. Finally, he places some arbitrary golden stars as rating against the bike in a magazine. What does the reader feel?

You'd want to snap - "Who cares about small numbers, just believe it and ride it!". Yeah, that's alright. But as bicycles get more expensive and new inventions border on that which is ridiculous, when bold claims ask for a lot of money in exchange, a customer would surely not mind knowing if there's true value in these claims or if there's some sort of daylight robbery going on.

Zertz - A marketed viscoelastic insert for reducing vibration. It comes standard on many of Specialized bicycles and cannot be removed or replaced.

One of those claims involve the relation of bicycle design with vibration reduction. For example, some years back, we saw Specialized incorporating an elastomer insert into their bikes at specific locations that supposedly "soaked up" the road chatter. Others have marketed frames and forks with special, curvy shapes that implied they're somehow better at vibration reduction, power transfer etc etc. Note that there is zero published technical evidence backing up the claims, yet people are quick to side with one brand or the other because of personal feelings.

This picture shows the harmonic tuned mass damper marketed by Bontrager as the Buzzkill Damper. This particular one was seen at times on Stuart O'Grady's bikes at the Paris-Roubaix. More on this can be found here.

One of the recent examples is Museeuw's biocomposite bike, a medley of organic flax and carbon fiber made in Belgium through a patented process that I've written about in the past. Their marketing strategy seems to be to make people believe that there's something really magical about its vibration dampening characteristics compared to competitor's bikes. Interestingly, they have joined hands with the materials engineering department at the University of Ghent in a partnership to do the R&D work. Apparently, one of the deliverables from the University would be an objective study of the bike's vibration dampening characteristics so that they can be presented to customers with commercial interests.

Recently, the 3D plot you see below was leaked out to the public on the internet after a Museeuw press launch. How it got leaked is a story you need not worry about. Anyway, the plot came directly out of one of the studies on the flax-carbon bike done by an individual named David Luyckx.

Fig 1 : This plot shows Damping Percentage vs Vibration Frequency vs Time for a Museeuw MF5, measured using two accelerometers mounted on the bicycle. Vibration frequency is a function of mass of the vibrating body, here, the bicycle and rider. Little is known to us about the test equipment and instrument characteristics of the accelerometers used.

He then compared it to the characteristics of 3 other bikes tested in the same study :

Fig 2 : This plot shows a comparison of vibration dampening of a (left to right) Pinarello Prince, Willier Cento Uno, Cervelo R3SL and the MF5

Now in the automobile and motorcycle industry, there are some specific ISO standards you have to follow to measure dynamic comfort and whole body vibration while sitting in a vehicle. None, as far as I know, exist that describe what to do incase of a bicycle. So David Luyckx set out to design his own experiment.

After reading his brief test report to us at rec.bicycles.tech, the following things can be said about the nature of his ideas and his experimental setup :

What To Measure : Ride comfort while using the flax-carbon bike, by studying trends in vibration dampening in the same (histeretic dampening). Specifically, the transmissibility of vibration would be measured. In other words, if there was a way to measure and determine the difference between the loads that were introduced into the frame and the loads that the rider would experience, it could be determined how "comfortable" a bicycle frame was quantitatively.

Experimental Setup : From his limited test report, Dave told us that he mounted an accelerometer near the rear wheel hub which he believed would give him an idea of the loads coming into the frame. A second accelerometer positioned just below the saddle on the seatpost would get him a measure of the loads before the rider experiences them. The difference, according to him, is how much of the vibration pie the frame takes eats away.

Methodology : All four frames - Museeuw MF5, Pinarello Prince, Wilier Cento Uno and Cervelo R3SL - were tested 4 times each with 2 clincher type rims (high and low profile) and 2 tubulars (high and low). If his idea was correct, by this method, he would not see too much difference between different wheelsets since he was only looking at only the frame properties between rearstay and saddle points. The measurements were done using independent accelerometers at a measuring rate of 50 Hz. The accelerometers were synchronized before the test. This enabled him to obtain a frequency spectrum of 0 to 25 Hz at any given time after putting the datasets through a Fast Fourier Transformation (FFT). He claimed this particular test method is comparable to how construction workers are monitored for whole-body-vibrations during their work. So, for every 27-second interval, the FFT-algorithm was used to get a 2D frequency spectrum, i.e. "frequency vs. load" graph. By using the 27- second interval he could avoid any response delay of the frame when impacted. By comparing each individual 27-second frequency spectrum of the rearstay and seatpost at the same interval, he was able to construct the 3D graphs shown above which involved approximately 300 graphs put next to each other.

Results :Final results showed a margin of difference of vibration dampening less than 5%.

Interpretation Of Graph : A value of "0.8%" on the y-axis in Figs 1 and 2, according to Dave, signifies that 80 percent of the original load is being absorbed or dampened somewhere between rearstay and seatpost. So he claims that the MF-5 dampens around 70 percent of the original load whereas the Pinarello Prince in Fig 2 absorbs only 45 percent of the original load measured at the rearstay of its frame.

Now I have to commend the fact that someone in the industry is taking the first steps towards thinking about how to measure vibration. But I must admit this is a very challenging task. It would take a lot more to convince people that the above basic testing makes sense. The graphs above look colorful but is confusing to interpret in 3D. The 5% of difference from the flax can even be argued to be practically imperceptible to any rider. As of now, the testing does not account for how the vibration can be affected by the following :

1. Amount of monitoring and placement of accelerometers - Can bicycle vibration really be fully captured by just two accelerometers on the bike? And how does their specific placement and mounting affect the frequency spectrum?

2. Cushy tires and a saddle - If you let some air loose from your tires, what's the effect on vibration dampening? Tires have significant roles to play in this aspect. It is well known that racers in the grueling Paris-Roubaix lower their tire pressures to about 80-85 psi to ride on cobbles. They even bend their elbows and loosen their grips on the handlebars to a significant extent. Also, if you have a cushy seat, the force on a rider might be tiny yet the accelerations on the seatpost may be large.

3. Varying frame geometries and designs - All 4 bikes tested have different geometries and aesthetic features. What effect do that have on vibration transmission or dampening? Can you say for certain that a curvy chainstay has zero measurable effect?

4. Frame flexing - A frame design is, to some degree, known to have comfortable ride characteristics if some level of compliancy is incorporated into the design. This means that the frame can flex finitely in a particular direction to reduce shock transmission and then transfer back the potential energy by acting sort of like a spring. If the flax frame reduces vibration by flexing, this can involve high forces. So one could theoretically make a noodly little frame which is poor in power transmission but perhaps great at shock absorption. So the above study does not establish conclusively whether the claimed vibration dampening in the flax-carbon frame is infact from the vibration soaking capabilities of the flax-carbon material or because of the flexing of the frame due to the mechanical properties of the overall structure.

Infact, I did a little research on the stiffness characteristics of the MF5 flax bike to try and make sense of point number 4 above. The German Tour Magazine, an independent 3rd party testing agency for top end bicycles, tested a 56cm Museeuw MF5 a while back. This is the same bike shown in Figs 1 and 2. After some translation, here's what I believe I found :

Let's put this above table into perspective.

Early this year, the same independent magazine tested 27 top end carbon fiber bikes that you can buy for money. From the published test results, I calculated that the average torsional stiffness for those 27 bikes was on the order of 95.85 Nm/degree, the average bottom bracket stiffness was 55.77 N/mmand the average lateral stiffness of the forks of these bikes was43.81 N/mm. So compared to those averages, the flax MF5 bike appears to be 29% lower in torsional stiffness, 21% lower in bottom bracket stiffness and 10% lower in fork lateral stiffness. This isn't sensational in the market, especially for the price of the frameset alone, a whopping 5000 dollars.

However, that's not the point. Suppose its these low numbers of stiffness that's providing all the "vibration soaking effects" in the flax frame? This can be a valid correlation, why not? Afterall, we all know that an overly stiff bike is not comfortable for long rides.

I'm eager to know more from David's side of these investigations and how this develops for the future. However, it stands right now that what he's taken upon himself is a challenging scientific task. If the outcome of these studies are minute percentage differences of one bike over the others, then someone can easily lose sight and perspective of the scale of numbers. That must be kept in mind. Meanwhile, I would encourage him and others who're on the same boat to look at the automobile industry, especially that of motorcycles and also study ISO standards on how to go about setting up experiments and measuring whole body vibration while using a vehicle.

If any of you are particularly interested in this topic, or is experienced in measuring vibration in your fields of work, please do write in to me with your thoughts here.

17 comments:

Thanks for your post. I think this is one of the more important issues with bike design & marketing.

Most reviewers are told what ride to expect prior to testing & have preconceived ideas of the ride characterists of different frame materials. So developing an unbiased repeatable testing protocol would be a huge leap forward.

I'm not sure how manufacturers will react, as I guess not all 'special' design features will be proved to be functional.

Ron their are a couple wild cards in the mix. That would be rider size, power, and weight. A bike that feels stiff to 140 lb rider may feel like a wet noodle to a rider that weighs 200 lbs. So the bike needs to be tuned to the individual rider. I think the future of frame building is custom composite frames for everyone. Here is how the process will work. "Just add money!"

A female frame mold will be rapid protyped based on the riders custom specifications. A ceramic male mold is then created from the female mold using "Advanced Cermacis Manufactering's" Aquacore Premium Ceramic. Then the carbon composite frame will be layed up over the ceramic core and cured in an autoclave. Once the frame is cured the water soluable ceramic core is dissolved away leaving the pure carbon frame now ready for finishing.

I was wondering would there be any way for the designers to do a modal analysis using computers? The only challenge I see here is how to tell the computer that the bike has a special composite material in it?

Everything around us is in frequencies. Light, sound, colors etc etc. If you turn on a red light in your room, that's a particular frequency. Now if you switch it off and turn a blue light on, that's another frequency. When we analyze a signal and see how it changes with frequency, it is called the frequency domain. If we analyze a signal with respect to time, we're in what's called time domain (this is usually how we analyze things. Example, when you tell someone you work 40 hours a week in your day job, your giving him some data in the time domain...40hours/week...40 hours/time).

Fast Fourier Transform allows you to move from time domain to the frequency domain by doing a set of mathematical operations on the time domain signal. We do this because there are many things that could be understood in the frequency domain that you cannot understand or analyze in the time domain. When you do a frequency breakdown of the signal, you're basically breaking down the initial time domain signal into individual component signals that "SWITCH ON" at a particular frequency. All these signals make up the initial signal. Its cool to analyze that!

Mr. Fourier was a French mathematician who proved all complex wave forms could be broken down into their individual frequency components mathematically. We use his math to convert time domain signals to frequency domain signals. Since this is a complex procedure to do by hand, there are computer softwares that do all the transformation for us. This is called the FFT operation. Hope that gave you atleast a basic appreciation for this jargon. Its difficult, I know.

An extra little precision. FFT just refers to any computational algorithm used to take a data set and move between time and frequency domains without actually having to do the detailed calculus involved of the DFT - this is great because experimental data sets don't always obey easily fittable functions nor can or do you want to spend time curve fitting in situ (something that is fairly perilous anyway).

As to the issue at hand, I think this is a great topic and a very well written post. I'm struck by the complete lack of some common engineering sense in the cycling industry and in the consumer.

I'm really amazed how much "magic" and black art the industry has managed to surrounded bike design with regards to stiffness and comfort - all in the name of spoon feeding marketing to the hapless consumer.

The engineering system itself might be quite complex, but the basic knowledge behind what needs to be done is not. Without being able to throw in a whole slew of "large displacement" dynamic elements that makes suspension systems comparatively robust it's high time road cyclists accept that there is only so much you can do and that any of it comes at a performance cost.

From a structural standpoint having a cushy seat and flexy frame generally will translate to a softer ride and that should be accepted basically as the gospel IMO, something that can probably be rationalized with a simple lumped spring mass damper system (mass of rider + bike), some net vertical compliance of the frame, etc. You're trading power transfer for that comfort every time though, point finale.

Other tricks, like zerts which I figure work on the basis of acoustic impedance mismatching and hysteresis might do something but I've yet to see any actual data on how much they do.

I think the biggest hoax out of all of this is that I'd bet tire pressure (which is quite simply adding more compliance and a touch of damping) and the wheel set probably does more than switching frames and that riding technique, proper position and a perfected bike fit are all dramatically more important to your long term comfort than a frame ever will be after a certain price point.

With regards to bike reviews, I think that any review that does not put quantities (ie 99% of all of the reviews of anything out there) are absolutely useless. Save your money and time and use those to test ride yourself - your LBS ows you at least that much for your continued patronage.

Jaycee : I would love to read Bontrager's ideas. Could you please link the Wayback archives story here, if you can? Besides, I did not imply frame flex totally 'absorbs' pedaling energy. I mentioned how it acts like a spring in the article. Springs are shock absorbers to a degree because they have a "K" value and a damping "C" value. Now whether it gives some of it back into positive pedaling motion is something to be discussed. Intuitively, I guess one could argue that it is.

Jason : You're absolutely right. If there was an Advertising Standards Authority for cycling, perhaps we wouldn't have seen so much of the black art you mention.

I hate to state the obvious, but all materials have natural damping in them or else they would continue to vibrate for ever. Superior damping characteristics was one of the selling points of magnesium frames when they first came out.

http://www.mgbiker.20m.com/mg.htm

Also check out Elastic hysteresis as I believe it explains the Bontrager phenomena.

I find that analysis highly suspect. The writer performs a purely static analysis at various points in a cycle, demands the reader ignores all those pesky complex forces and then magically comes up with the notion that the spring is giving back to the upward pedal stroke exactly as much as it took and perfectly in phase and in plane.

Sorry, but thats called bullshit where I'm from. What happens to all that torsion that is totally out of plane of the bearing surfaces?

The way I see the dynamics of the system is that any flex in the BB location causes the force vector of your pedal stroke to move out of plane of the chainring, thus lessening the effective torque you are applying and it is not immediately clear to me that the deflection provides an exactly compensatory restoring force because the force vectors on each side of the BB are actually different because the crankset is not symmetric.

Also, the only wheel flex that is not elastic is what goes on in the tires, and the tires will always be significantly more compliant and thus bear the bulk of the deformation even if you make your metal frame a touch more flexy